Key Takeaways
- A subsonic filter is a high-pass on the sub channel that blocks the very low content below the box's usable range.
- It protects the sub and frees amplifier headroom; it does not add or enhance bass.
- Ported boxes need one, set at or just below the port tuning frequency (a 32 Hz box wants about 28 to 32 Hz).
- Sealed boxes are self-protecting, so a subsonic filter is optional, set low (15 to 20 Hz) or off.
- Set too high, it cuts audible bass. Keep it near tuning for ported, low or off for sealed.
A subsonic filter is a high-pass filter on the subwoofer channel that blocks the very low frequencies below the box's usable range. Its job is to protect the sub and free up amplifier headroom, not to add bass. It matters most on ported boxes, which over-travel below their port tuning without one. This explains what it does and exactly where to set it.
This is a tuning detail that ties into the rest of the sub setup. See power handling for why over-excursion kills subs, the enclosure guides for box tuning, and the gain guide for the rest of the amp setup.
What is a subsonic filter?
A subsonic filter, sometimes called an infrasonic filter, is a high-pass filter applied to the subwoofer signal. It sets a floor: below the frequency you choose, the filter rolls off the signal so the sub does not try to reproduce it. That low content is things like rumble, warped-record noise, and the deep infrasonic energy in some recordings, frequencies below what your box can usefully make.
Here is the correction to a common myth: a subsonic filter does not enhance or add bass. It removes content. What it does is stop the sub and the amp from wasting power and cone travel on frequencies the box cannot reproduce, which protects the driver and frees headroom. That freed headroom is why a correctly set subsonic filter can let you turn the usable bass up cleanly, but the filter itself only ever removes.
Why do subwoofers need a subsonic filter?
Because of what happens below a ported box's tuning. A ported enclosure uses the port to control the cone around the tuning frequency. Below that tuning, the port stops loading the cone, the air in the port no longer resists it, and the sub suddenly moves very far on any low content that gets through. That uncontrolled travel is exactly the mechanical over-excursion that tears a sub apart, and it happens on frequencies you cannot even hear.
A subsonic filter stops that. By blocking the content below tuning, it keeps the cone from flying past its limits on infrasonic energy, and it stops the amp from spending power down there. It is the single most important protection setting on a ported subwoofer system.
Ported vs sealed: where to set the subsonic filter
This is the part that decides everything, and it depends on your enclosure.
| Enclosure | Subsonic filter | Set it to |
|---|---|---|
| Ported | Required | At or just below the port tuning frequency (a 32 Hz box wants ~28 to 32 Hz) |
| Sealed | Optional | Low, around 15 to 20 Hz, or off |
| Bandpass | Required | Near the box's low tuning point |
The logic is simple. A ported or bandpass box has a frequency below which the sub unloads, so you set the subsonic filter right around that point to protect it. A sealed box uses the trapped air as a spring that already limits travel at low frequencies, so it is self-protecting and the filter is optional, used only to trim rumble.
How to set your subsonic filter
You set the subsonic filter on the amplifier, usually a dial next to the low-pass and bass-boost controls, or in a DSP. Many carried monoblocks include it: the Image Dynamics SQ1200.1D and the US Acoustics Big Ben both have adjustable subsonic and low-pass filters built in.
Start from your box tuning. For a ported box, set the subsonic dial at or just below the tuning frequency. If you are unsure of the tuning, or want to verify, play a low sweep at moderate volume and watch the cone: set the filter just above where the cone starts moving wildly without making sound. Set the subsonic before you finish the gain, and keep bass boost modest, because boost stacked on top can undo the protection. Here is a walkthrough of dialing subsonic and low-pass together:
Common subsonic filter mistakes
- Setting it too high. Push it up into the audible bass and you thin out your low end. Keep it near tuning, not up at 40 or 50 Hz.
- Running a ported box without one. This is how ported subs fail on infrasonic content they never should have tried to make.
- Confusing it with the low-pass. The subsonic blocks the bottom; the low-pass blocks the top, around 70 to 90 Hz. A sub uses both.
- Expecting it to add bass. It removes content. Any extra output you hear is freed headroom, not new bass.
Frequently Asked Questions
What is a subsonic filter?
Do I need a subsonic filter?
Where should I set my subsonic filter?
Does a subsonic filter increase bass?
What is the difference between a subsonic filter and a low-pass filter?
Can a subsonic filter be set too high?
Does a sealed box need a subsonic filter?
Where to go next
The subsonic filter is one piece of a safe sub setup. Pair it with a correctly matched amp and gain: see power handling for why over-excursion matters, the enclosure guides for finding your box tuning, and the gain guide for the rest of the amp. Shop mono amplifiers with built-in subsonic controls.
Not sure of your box tuning or where to set the filter? Contact us. We set the subsonic and gain by measurement on every system before it leaves the shop.
About the Author
Scott Welch is a Multi Time IASCA National and MECA World Sound Quality Champion, an active SQ judge since 2019, and the owner of Audio Intensity in Tullahoma, Tennessee. He cuts every Proline X enclosure on the shop's CNCs and tunes every customer system before it leaves. Audio Intensity is the original US importer for Goldhorn DSP and an authorized dealer for Prodigy, Crescendo, Image Dynamics, Wavtech, Tru Technology, and more.
